1. Field of the Invention
This invention relates to the extraction of magnesium chemicals from brucite bearing minerals. In particular, the invention is concerned with the selective extraction of the brucite mineral fraction from chrysotile asbestos by chemically opening the chrysotile asbestos fiber bundles and recovering the magnesium, which is thereby leached in a relatively pure form as a magnesium compound.
2. Description of the Prior Art
Brucite is a magnesium bearing mineral having the chemical formula Mg(OH).sub.2. Its high magnesia content (69% MgO) and high chemical reactivity make it a desirable source for magnesium chemicals. It is frequently found in association with other magnesium bearing minerals, such as magnesite and chrysotile. When brucite is associated with chrysotile asbestos fibers, it is an undesirable impurity because it imparts undesirable physical and chemical properties to the chrysotile asbestos fibers and because its presence in the deposit increases the severity of the processing required to extract the chrysotile asbestos fibers. Brucite is more brittle and chemically more reactive than chrysotile. The high chemical reactivity of brucite leads to a less stable chrysotile fiber in acid environments.
Conventional processing of chrysotile asbestos involves the physical crushing and sizing of the chrysotile bearing rock in order to open the fiber bundles and release the fibers. Chrysotile fibers have several mineral variations which relate to the ease of processing and to the amount of brucite present as a binder material in the bundles of chrysotile fibers. If the chrysotile fiber bundles are oriented perpendicular to the axis of the mineral vein, they are described as "cross" fibers. If the chrysotile fiber bundles are oriented parallel with the vein of the mineral deposit, they are termed "slip" fibers. Slip fibers tend to have brucite associated with the fiber bundles in such a manner as to act as a binder for the chrysotile fibers. The presence of the brucite in the slip fiber asbestos results in a more severe processing requirement to extract the chrysotile and a lower value fiber product. It would be desirable if an effective means could be found to separate the brucite from the chrysotile without damaging the chrysotile fibers.
Prior art in the extraction of magnesium from serpentine ore sources is limited to processing which is either severe enough to extract the magnesium from both the chrysotile and brucite or ineffective for selectively leaching magnesium from the brucite present in chrysotile. According to Canadian Pat. No. 1,034,385, serpentines have been treated with hydrochloric acid to extract about 95% of the available magnesium in the ore, and this extracted magnesium is then recovered as a caustic calcined or reactive magnesia. Another process for the extraction of magnesium from serpentine wastes is based upon the heat treatment of both chrysotile and brucite minerals with a mixture of crystalline ammonium sulfate in order to convert the magnesium in the minerals to a water soluble magnesium sulfate. The choice of ammonium sulfate as part of the mixture in the heat treatment is unique in that other ammonium salts lead to magnesium salts that decompose at the temperature of heat treatment or lead to undesirable side reactions. The magnesium sulfate is leached from the heat treated material with water and the soluble magnesium is recovered as a magnesium compound. Because of the severity of the reaction conditions necessary to form the magnesium sulfate in this process, the chrysotile mineral is destroyed either totally or in part during the heat treatment. Aqueous carbon dioxide is known to leach magnesium from brucite and magnesite minerals but the process is such that carbon dioxide leaching is not suitable for materials having low brucite or magnesite contents.